The present invention relates to a method and a device for processing a solution, melt, suspension, emulsion, slurry or solids into granules of a classified size.
Fluid bed granulation or fluidized bed granulation is a technique used in particulation of melts, solutions, slurries, emulsions, suspensions or solids, for instance in the fertilizer and food industries.
A fluid bed granulation process combines several sciences and technologies. To operate a fluid bed granulation plant properly, knowledge of melt and solution chemistry, crystallization properties, total mass- and energy balance, mass- and energy transport, particle- and granulometry balance, fluid dynamics and fluidization technology are required.
To design and operate these plants is difficult due to the fact that the mass balance, energy balance and granulometry balance must be set correctly to give the right performance with regard to capacity and quality. Each of the balances can not be set independently as most of the control parameters available to operators and designers affect all three balances. The balances expressed as a limited and simplified set of equations will also have several solutions, where the optimum or best solution depends on the chemical and physical properties of the product system, product quality and cost of utilities and other input factors.
Different salt systems have different solubilities and different heats of crystallization. In fluid bed design, these differences give a variety of design parameters and settings for air flow and temperature, recycle flow and temperature, melt temperature and concentration. The most important factor for the fluid bed granulation process is the control of the liquid phase together with the overall energy balance and granulometry balance through the particle growth and the production of seed particles.
A seed particle is defined as a particle too large to be carried out with the exhaust airflow through the granulator, large enough to prevent being agglomerated with other particles, and smaller than the desired product size.
In a conventional fluid bed granulation process, the size distribution of the produced granules has been controlled by recycling a certain fraction of undersized granules and crushed oversized granules to the granulator. This eases the operation and the flexibility of the process, making it possible to handle various systems and granulometry, and still be able to control the conditions in the fluid bed (i.e., the liquid phase and the crystallization evaporation rate). The fact that a fluidized bed granulator operates as a total mixed reactor has further supported the robust design and operational philosophy.
An excessive recycle stream, 0.5 to 2 times the product flow, carrying an excess of seed granules and mass flow, limits the influence of and sensitivity to other operating parameters. This has limited the interest in developing classifying granulators. Fluid bed granulation processes are sensitive to the number of seed particles produced, as agglomeration is undesired and should be avoided from a product quality and operating stability point of view. Agglomeration creates particles with lower crushing strength and it is difficult to use agglomeration to control the particle balance without increasing the recycle ratio to 3-7. A robust design with an excess recycle stream as an important control parameter, has been preferred by the industry. A low recycle flow is only possible.
A classifying fluid bed granulator is defined as a granulator that is able to discharge the product that is the largest granule fraction contained in the bed. The product continuously has a granule size which is larger than the granules in the granulator. The efficiency of the classification depends on the methods applied for classification and the size differences handled by the bed. A classifying granulator will, in a dynamic process, give a shorter retention time for the desired product fraction of large granules, thus giving a longer retention time for the smaller granules, enabling them to grow more before reaching the product size and be discharged. A classifying granulator will also be able to perform as an ideal plug flow reactor, given a feed of uniform seed material. Screening and recycling of the granules in conventional fluid bed granulators is always done outside the bed as, for instance, described in U.S. Pat. No. 4,219,589.
Building mechanical screening and crushing into or close to the fluid bed granulator is described in DE 3248504-C2, but has-been seen as not advantageous from an operational point of view.
However, U.S. Pat. No. 4,790,487 describes a continuous granulator where screening and recycling is done in an adjacent unit being a combined screw conveyor and fluid bed. The patent describes an apparatus comprising a granulator body for continuously processing powdered materials into granules and a screw conveyor for discharging the produced granules from the granulator body. The screw conveyor includes means for pneumatically classifying the produced granules while they are being conveyed. The patented principle will only be able to separate and recycle the dust or fine particles from the discharge flow. The classification efficiency in the method is based on the difference in escape velocity between the large onsize granules and the dust fraction, and will not be able to separate 1-2 mm particles out of a mass containing 1-5 mm particles. The bubble formation and slugging will create a flow of particles of all sizes between 1-5 mm back: to the granulator.
Internal segregation effects in fluidized, spouted and moving beds have been described in several publications. The effects of air velocity and bubble breaking constructions inside the bed have produced documented effects achieving a particle size difference between top and bottom in a single bed compartment. In “Powder Technology 98” (1998) 273-278, the effect of horizontal baffles are described and documented.
The bed design with the internal baffles results in a single chamber high bed with a subsequent high pressure drop. The total bed movement is reduced by both the baffles and the geometry, and the bed achieves a lower capacity because the heat and mass transfer requires turbulence and particle movement.
Another disadvantage, making these principles less useful, is the lack of horizontal classification. With a vertical classification effect only, size and shape of the granulator is limited in area to bed height ratio, and is therefore tested in a single chamber only. Horizontal baffles placed in the single granulator chamber, as described in WO 97/02887, is also seen as a practical disadvantage, as it gives less freedom to install spraying nozzles.
A significant disadvantage with a conventional but robust fluid bed process is the high investment costs in screens, crushers, dissolving units, dryers, coolers, intermediate storage and solid material transportation inside the plant. This requires large buildings and expensive steel constructions to enable an operable layout. Each mechanical and electrical item requires design, engineering, commissioning, spare parts, monitoring, maintenance, cleaning and attention from operators. In a corrosive environment due to salts and humidity, the quality of materials increases the investment cost further. The number of mechanical items increases the failure rate and risk of expensive down time.
Furthermore, operation of these granulation plants requires frequent stops for maintenance of mechanical and electrical equipment and cleaning of process equipment. Recovery of washing water and extra space inside the plants for maintenance activities further increases the cost for constructing and operating such plants. Reducing the recycle flow by optimizing the seed production and controlling the crystallization and solidification process has given some competitive advantages for the best processes.
Thermodynamically it is possible to design a fluid bed process with no recycling of cooled or heated granules outside the fluid bed. An optimum heat balance over a fluid bed granulator can be achieved by changing air temperature or air flow. A relatively large air flow is required anyway for the fluidization itself. The heat balance can alternatively be solved by internal cooling or heating in the fluid bed itself.
However, to operate a fluid bed granulation process without recycling material requires a control of the granule growth in a different way than in conventional beds mentioned above. Granule growth and product granulometry in conventional beds are a function of size distribution of feed or crushed recycled material, the feed to melt ratio, and classifying effects in the fluid bed or granulator. Conventional beds have low classifying efficiency, operating almost like, a total mixed flow reactor. The product from a total mixed reactor will consist of a mix of fresh undersized feed and matured larger particles. Even with an ideal plug flow reactor, the product is largely dependent on the size distribution of the feed or recycled material.